1. Field of the Invention
The present invention relates to minimizing defects in the components produced by an extreme ultraviolet lithography (EUVL) system, and more specifically, it relates to a method for repairing defects in the multilayer coating of an EUVL reticle.
2. Description of Related Art
The reticle blank to be used in an extreme ultraviolet lithography (EUVL) system is expected to consist of a thick substrate coated with a reflective multilayer film. This multilayer film is subsequently covered with a patterned absorber layer to form the EUVL reticle. See C. W. Gwyn, R. Stulen, D. Sweeney, and D. Attwood, xe2x80x9cExtreme Ultraviolet Lithographyxe2x80x9d, J. Vac. Sci. Technol. B 16, 3142 (1998). When the reflective multilayer coating is deposited on substrate imperfections such as particles, pits or scratches, the layer structure of the multilayer film is perturbed, resulting in mounds or depressions in the contour of the layers. See P. B. Mirkarimi, S. Bajt, and D. G. Stearns, xe2x80x9cMitigation of Substrate Defects in Reticles Using Multilayer Buffer Layersxe2x80x9d, U.S. patent application Ser. No. 09/454,715 now U.S. Pat. No. 6,319,635, incorporated herein by reference. These defects produce unacceptable intensity variations in the lithographic image when they exceed a lateral dimension of about 30 nm, and a height variation of more than about 1 nm. See T. Pistor, Y. Deng, and A. Neureuther, xe2x80x9cExtreme Ultraviolet Mask Defect Simulationxe2x80x94Part IIxe2x80x9d, J. Vac. Sci. Technol. B, to be published in the November/December 2000 issue.
The above-incorporated patent application proposes using buffer layers to mitigate the growth of multilayer phase defects from small substrate particles and features that cannot be detected by standard inspection tools. The buffer layer concept exploits the smoothing properties of specially designed films to reduce the size of the substrate perturbations to a level at which they will not nucleate a critical (i.e., printable) phase defect in the multilayer reflective coating. Although this approach should greatly reduce the population of phase defects, it must be expected that a few critical phase defects will always be present. These could arise, for example, from the contamination of the top surface of the buffer layer. Hence it is essential that a process be developed for repairing a small number of phase defects in the Mo/Si multilayer reflective coating on a EUVL mask. The problem with repairing the multilayer reflective coating is that it is fundamentally incompatible with the current mask repair technology. The current technology involves repairing defects in the metal absorber layer by locally removing or depositing material; however, the repair of the multilayer film must consist of correcting the local deformation of the layers within the film.
U.S. Pat. No. 5,272,744, titled xe2x80x9cReflection Maskxe2x80x9d, by M. Itou, H. Oizurm, and S. Moriyama, granted Dec. 21, 1993 (Itou et al.) describes a special reticle for x-ray and extreme ultraviolet lithography that is provided to facilitate the repair of multilayer defects. This reticle comprises two multilayer film stacks separated by an Au layer and is in contrast to the conventional reticle design incorporating patterned absorber layers on a multilayer film or the other design of a patterned multilayer on an absorber. (See also U.S. Pat. No. 5,052,033 titled xe2x80x9cReflection Type Maskxe2x80x9d by T. Ikeda et al.) There are some disadvantages to the Itou et al. approach, including (i) their reticle is more difficult and expensive to fabricate than other designs, (ii) the introduction of the Au layer will likely introduce additional roughness in the reflective overlayer, reducing the reflectance and throughput of the lithography system, (iii) their repair process is not a local one and involves covering the entire reticle blank with resist, etc., which could lead to new particulates/defects, and (iv) it is uncertain whether their method will work in a practical sense since it requires extreme control of the Au deposition and various etching processes so that a phase defect does not result from the multilayer defect repair process.
It is desirable that techniques be provided for the repair of multilayer film defects in reticles for use in extreme ultraviolet lithography.
It is an object of the present invention to provide a method for repairing defects in a multilayer coating that is deposited onto a reticle blank used in an extreme ultraviolet lithography (EUVL) system.
Other objects of the invention will be apparent to those skilled in the art based on the teachings herein.
A localized energy source is used for depositing energy in a multilayer coating of a reticle blank. The energy is deposited in the vicinity of the defect in a controlled manner and with high lateral spatial resolution. This can be accomplished using a focused electron beam, focused ion beam, focused electromagnetic radiation or through direct contact with an electrode. If the multilayer comprises an appropriate material combination, the absorbed energy will cause a structural modification (e.g., interdiffusion at the layer boundaries), producing a localized change in the film thickness. By adjusting the energy dose, the change in film thickness can be controlled with sub-nanometer accuracy. Localization of the energy deposition controls the lateral spatial resolution of the thickness modification. The film thickness is adjusted locally to correct the perturbation of the reflected field. For example, in the case where the structural modification is a contraction of the film, the repair of a defect consists of flattening a mound or spreading out the sides of a depression. The defect repair can be applied directly to the reflective multilayer coating or to a buffer layer consisting of a multilayer film deposited below the reflective multilayer. Finite element simulations for the case of an electron beam impinging on a Mo/Si multilayer film indicate that this is a viable defect repair technique for EUVL reticle blanks.
This invention has the potential to impact current extreme ultraviolet lithography (EUVL) system development and to impact government programs such as ASCII. There is a strong commercial driving force for increased miniaturization in electronic devices, and hence an extreme ultraviolet lithography (EUVL) tool has significant commercial potential. To be economically viable, this technology requires a nearly defect-free reticle. Commercial integrated circuit manufacturers currently rely on defect repair techniques to obtain reticles with sufficiently low defect densities; however, these repair techniques cannot be applied to EUVL reticles.